Examples

When one syllable of a word ends with a vowel, and the next syllable begins with the same vowel, the hyphen is placed between the syllables to indicate that the two vowels do not form a diphthong, that is, that they should not be pronounced together.

One exercise that I am recalling required placing a sheet of paper in front of our mouths while speaking -- it wasn't supposed to move -- and then there was the problem of diphthong (a special problem for many with "southern" accents) ... ahh, the memories!